1. Field of the Invention
The present invention relates to a power supply apparatus whose input source is a commercial AC power source, and an image forming apparatus including the power supply apparatus.
2. Description of the Related Art
There are widely used power supply apparatus, which are to be mounted in various electronic equipment, and are supplied with power from a commercial AC power source so as to output necessary DC voltages to be used in the electronic equipment. As those power supply apparatus, there are known apparatus equipped with a switching converter unit. In the power supply apparatus equipped with a switching converter unit, a rectifying and smoothing circuit is disposed downstream of a commercial AC power source input unit so as to generate a DC voltage. Then, the switching converter unit (DC-DC converter unit) chops the DC voltage by on-off operation of a switching element and then smoothes the chopped voltage so as to ultimately output a stable necessary DC voltage.
Hitherto, electronic equipment such as a copier and a printer usually uses a dedicated power supply apparatus whose input voltage is an AC 100 V system (for example, AC 100 V to 120 V) for Japan and North America, or an AC 200 V system (for example, AC 220 V to 240 V) for Europe. In general, an aluminum electrolytic capacitor is often used in a rectifying and smoothing unit of this dedicated power supply apparatus. In many cases, as the aluminum electrolytic capacitor, an optimal component is selected for each region of the AC 100 V system or AC 200 V system. For instance, a capacitor having a withstanding voltage of 200 Vdc is used for the power supply apparatus of the AC 100 V system, and a capacitor having a withstanding voltage of 400 Vdc is used for the power supply apparatus of the AC 200 V system.
When a voltage equal to or higher than the withstanding voltage is applied to the aluminum electrolytic capacitor, the internal pressure increases because leakage current increases. Then finally, an explosion-proof valve (hereinafter simply referred to as “value”) acts so that high pressure gas containing the electrolyte bursts out. This phenomenon is called “valve-open”. Because the electrolyte used in the aluminum electrolytic capacitor has conductivity, the electrolyte burst from the aluminum electrolytic capacitor may cause a short circuit between components or between circuit patterns, resulting in a circuit malfunction.
The power supply apparatus needs to be protected from break-down even when a voltage equal to or higher than the withstanding voltage of the aluminum electrolytic capacitor is supplied from the commercial AC power source. For this reason, it is desired not to cause the valve-open of the aluminum electrolytic capacitor. Hitherto, there are proposed means for protecting the power supply apparatus from application of an overvoltage from the commercial AC power source. For instance, Japanese Patent Application Laid-Open No. 2009-219316 and Japanese Patent Application Laid-Open No. 2009-284561 propose protecting means for turning off a switching element (such as a relay or a triac) when the detected voltage is a predetermined value or higher by using an input voltage monitoring unit for monitoring the input voltage disposed in the switching converter unit. In addition, Japanese Patent No. 4612855 proposes means for detecting a voltage applied to the aluminum electrolytic capacitor for smoothing the input voltage and supplying an excessive current to a circuit so as to blow out a fuse of an input portion when the detected voltage is a predetermined value or higher.
Electric power environment is usually poor in developing countries compared with developed countries. As a result, an overvoltage is often applied to the electronic equipment in developing countries. Therefore, it is desired to adopt a resettable protection unit which can protect the electronic equipment against application of an overvoltage and can return to a power suppliable state when the overvoltage state is canceled so that the overvoltage protection is released. In addition, requirements for saving energy in electronic equipment are increasing day by day, and it is required to achieve both the resettable protection against application of an overvoltage and the energy saving.
However, the related art techniques described above has a problem that it is possible to achieve one of the resettable protection against application of an overvoltage and the energy saving, but it is difficult to achieve both of them. For instance, the power supply apparatus described in Japanese Patent Application Laid-Open No. 2009-219316 and Japanese Patent Application Laid-Open No. 2009-284561 have a resettable protection unit against application of an overvoltage. However, because an overvoltage protection circuit is mounted, power consumption for turning on the switching element and power consumption in the input voltage monitoring unit are necessary, and hence standby power consumption (in a sleep mode or the like) is increased. Therefore, in a standby mode, the power consumption for turning on the switching element and the power consumption in the input voltage monitoring unit become larger relatively to power consumption of the entire electronic equipment and cannot be neglected for achieving energy saving. In addition, because the protection unit disclosed in Japanese Patent No. 4612855 blows out the fuse when an overvoltage is applied, the equipment cannot be returned to the original state when the overvoltage state is canceled.